No.
10 - Winter/Spring 1988

Pluto:
The Farthest Planet (Usually)

Pluto, the ninth
planet in our solar system, was not discovered until 1930 and remains a very
difficult world to observe because it's so far away. At an average distance
of 2.7 billion miles from the Earth, Pluto is a dim speck of light in even the
largest of our telescopes. It takes almost 249 years to make one swing around
the Sun, in a long looping orbit that takes it above and below the path of the
other planets. (See the accompanying box for more on its unusual path.)

What
Would It Be Like on Pluto?

While astronomers
don't yet know many details about the landscape on Pluto, we do know that it's
cold and dark out there. On average, Pluto is nearly 40 times as far from the
Sun as we are. From that great distance, the Sun would look like a single, brilliant
point of light (it would be only about 1/40th as big as the full Moon is in our
sky — too small to appear as a disk.)

During the day,
that tiny point illuminates the ground on Pluto with only 1/1500th the intensity
of sunlight we receive on Earth. (That's still far from being "dark,''
though: the Sun's light output as seen from Pluto is about 250 times the light
we receive from the Moon when it's full.) As you might expect, Pluto isn't warmed
much by the Sun; astronomers estimate its surface temperature to be more than
200 degrees below zero, Celsius. This is a temperature so cold that skin would
be as brittle as glass — and that some materials we're familiar with as gas
or Earth (such as methane) would freeze solid.

New
Discoveries in the Last Decade

Despite its great
distance, Pluto has given up some of its secrets to careful study recently. Even
though no spacecraft has investigated Pluto (and none is scheduled to), a happy
circumstance is in part responsible for the accelerating pace of discoveries:
Pluto is closer to us in the last two decades of the 20th century than it has
been for the last 200 years (or will be for the next 200). Along with the great
sensitivity and sophistication of today's astronomical instruments, this makes
investigating Pluto from Earth less difficult now.

In 1978, for example,
a relatively large satellite was discovered around Pluto and named Charon —
and during the last half of the 1 980's, a rare alignment has made it possible
to study Pluto and Charon much more effectively than ever before. For a brief
period once every 124 years, observers on Earth are in a position to see Charon
pass directly in front of and behind Pluto in its 6.4-day orbit around the planet.
This "eclipse season'' began in 1985 and will end in 1990; there won't
be another one until the 22nd century — and not until the 23rd century
will one happen when Pluto is this close. By making careful measurements of
the eclipses, astronomers have made great progress in understanding the sometimes
strange and puzzling properties of Pluto and its satellite.

Pluto
is Not the Ninth Planet

Ever since 1930, school
children have memorized the nine planets in order: Mercury, Venus, Earth, Mars,
Jupiter Saturn, Uranus, Neptune, and PIuto. But between January, 1979 and March,
1999 that order is not correct. Pluto's eccentric (ellipse-shaped) orbit has brought
it inside the orbit of Neptune, making it the eighth planet for two decades. Pluto's
unusual position makes "What is the ninth planet?'' a great trivia question
for a while.

Just in case students
begin to worry that someday Pluto might collide with Neptune as the smaller
planet crosses the orbit of the larger, you can reassure them. The two orbits
are tilted relative to one another by 17 degrees) in such a way that
they never actually "touch.'' (Imagine two enormous, slightly elongated
hula hoops, one larger than the other. If the larger one is tilted relative
to the smaller one, you can imagine that the two points where the larger hoop
crosses through the smaller hoop's plane can be outside the smaller hoop.) Calculations
indicate that Pluto's and Neptune's orbits are never closer than 385 million
kilometers to one another and that (at least for the past several million years)
the two planets themselves have never been closer than two and a half billion
kilometers to each other.

Some
Pluto History

The eighth planet
from the Sun, Neptune, was discovered in 1846 at precisely the position that two
mathematicians one English and one French—had predicted it would be. John C .
Adams and Urbain J.J. Leverrier had calculated that a new planet must be responsible
for the fact that the motion of the seventh planet, Uranus, was not in accord
with the laws of planetary orbits (derived from Newton's theory of gravitation).
In essence, they reasoned that some significant body, more distant than Uranus,
must be adding an extra influence on its movement. Astronomer Johann Galle at
the Berlin Observatory then found Neptune on the very first night he began to
search!

As you can imagine,
this success naturally led astronomers to wonder if even more distant planets
could be found in this way. Unfortunately, Neptune takes 165 years to complete
its orbit around the Sun and, as the 20th century arrived, it had only moved
through a small part of its orbit. This made it very difficult to calculate
the degree to which it was not orbiting "properly'' and other bodies might
be disturbing it. Nevertheless, a number of astronomers attempted to calculate
where a ninth planet might be from possible "oddities'' in the motions
of both Uranus and Neptune.

The best-known
predictor of a ninth planet was a wealthy Bostonian named Percival Lowell, whose
advocacy of the possibility of life on Mars had brought him fame in the 1890's
and 1900's. Lowell built one of the finest observatories in the country just
outside Flagstaff, Arizona, in part to pursue his dreams of Martians and new
planets. Neither could be found by the lime of his death in 1916.

Nevertheless, Lowell's
dream was kept alive by the trustees and staff of the Lowell Observatory, and
in the late 1920's Lowell's brother Lawrence (who was president of Harvard University
for many years) gave $10,000 to fund a special wide-angle 13-inch telescope
to make a thorough photographic search for a ninth planet.

Because the large
photographs typically taken by such telescopes are full of huge numbers of stars,
it was clear to the astronomers at Lowell that many months and perhaps years
of careful photography — and very patient inspection of the photographs —
might be required to find their planet. Someone would have to sit night after
night in a cold, open dome and then spend the days searching the negatives for
a moving minuscule black dot among the thousands of stationary ones. It occurred
to them that it would perhaps be best to train a good amateur astronomer who
could dedicate himself to the task.

Two
telescopic photographs of Pluto (the dot at the arrow's tip) taken 24 hours
apart. By carefully checking its position against the background star pattern,
you can see that it has moved a little from the first picture (left) to
the second (right). (Photographs courtesy of Yerkes Observatory.)

Pluto's Discovery

By happy coincidence,
a young farm boy from western Kansas wrote to the director of Lowell in 1928,
enclosing some careful drawings of Jupiter and M ars that he had made while
observing with his home-made 9-inch telescope. A few months afterwards, 22-year
old Clyde Tombaugh boarded a Santa Fe train at Larned, Kansas and arrived at
Flagstaff 28 hours later to begin the search for what came to be called Pluto.

The typical photographic
image Tombaugh used — recorded on plates of glass to have a good permanent
record — might contain anywhere from 50,000 to 400,000 stars, galaxies, and
asteroids. How could the faint image of a distant planet be distinguished?

The trick was to
take two photographs of each region of the sky several days apart. Then the
pair of plates could be put side by side into a machine called a blink comparator,
in which the two images are quickly alternated (blinked) in a single viewer.
If an object is in the same place on both plates (as a star wouId be), no motion
will be seen as the plates are switched in the viewer. But if the object has
moved in the few days that elapsed between the taking of the plates (as a planet
would do), its image will "blink'' — shift back and forth against the
still background of the stars.

Even with this
machine, it was an enormous and painstaking task to cover the parts of the sky
in which planets were expected to be found. It took almost a year of searching,
but on February 18, 1930, while blinking two plates of a region in the constellation
Gemini, Tombaugh discovered a new planet not far from where Lowell had predicted
it wouId be — although much fainter than he had suggested. After confirmation
and careful study, the announcement was made on March 13th, the 149th anniversary
of the discovery of Uranus and the 75th anniversary of Percival Lowell's birth.
Eventually, following the suggestion of a British schoolgirl, the planet was
named Pluto, after the Roman god of the underworld. (Another point in favor
of the name was that the first two letters were Percival Lowell's initials!)

Pluto's
Moon

In 1978, astronomer
James Christy was examining some photographs of Pluto taken with the U.S. Naval
Observatory's telescope near Flagstaff — not far from Lowell Observatory. He
noticed that the image was slightly distorted — it bulged out on one side as
if Pluto had something next to it. After looking more carefully at photographs
that had been taken in the past, Chrisly was able to show that Pluto had a satellite
orbiting it at a distance of about 17,000 kilometers. He suggested the name Charon
— after the boatman who ferried the dead into the realm of Pluto in mythology
— and the name was made official by the International Astronomical Union in 1985.

Charon orbits around
Pluto in about 6.4 Earth days, which is exactly the time it takes Pluto to spin
around once on its axis. In other words, the day and month on Pluto are the
same length, a situation which only holds for Pluto and Charon out of all the
planets and satellites in the solar system.

However, there
are many artificial satellites around Earth for which this is also the
case. Called "geosynchronous'' satellites, these orbit above our planet's
equator at exactly the same rate as the Earth spins — so they seem to "hover''
(22,000 miles up) over the same place on Earth all the time. Most satellites
which are used to relay television signals are of this variety, so that receivers
on the ground ("satellite dishes'') can just be pointed at one direction
in the sky and then left alone — the geosynchronous satellite will always appear
to be in just about the same place in the sky.

Pluto's moon Charon,
then, can be thought of as a "Pluto-synchronous'' satellite — it would
always be in the same place in the sky if you were on Pluto. It would be an
eerie sight, too. Even though Charon is only about a quarter the size of our
own Moon, it's so close to Pluto that it would look bloated to Earth-trained
eyes: it would be eight times as wide as the full Moon appears on Earth,
covering 64 times as much sky. (It would be dim, though — remember that
it is reflected sunlight that makes a moon shine, and sunlight out there
at Pluto and Charon is very dim.) Hanging in the same place in the sky, day
and night, it would go through its full cycle of phases from one Pluto day to
the next (that is, every 6.4 Earth days) . During the "eclipse season''
there would be an eclipse of the Sun for a while every day (while Charon blocks
the Sun from view) and an eclipse of the moon at night (while Charon moves through
Pluto's shadow).

The
Latest Discoveries

Pluto is so far away
and difficult to see that not very much could be pinned down about its detailed
characteristics until the "eclipse season'' began. Since no spacecraft is
scheduled to visit Pluto, this is our one and only chance for the foreseeable
future to understand what happens in the distant realm that (usually) marks the
edge of our planetary system. Astronomers at various observatories have been studying
the Pluto-Charon system intensely and a preliminary report on their work was given
at a recent meeting of the American Astronomical Society in Austin, Texas.

The diameters of
the two bodies could be measured accurately for the first time during the eclipses.
Pluto turns out to be only about 2300 kilometers across (1400 miles) — clearly
making it the smallest planet in the solar system. In fact, seven moons (including
the Earth's) are larger than Pluto.

Charon is about
1300 kilometers wide (or roughly 800 miles). While much smaller than our Moon's
3500 kilometer diameter, this is so large compared to Pluto — more than half
as big — that some astronomers are now speaking of the Pluto-Charon system
as a "double planet.'' The next closest thing to a double planet is our
own Earth-Moon system, but our satellite is only about one-fourth the diameter
of our planet. (It must be noted that these measurements of Pluto's and Charon's
diameters are not — and cannot yet be — precise. It would not be surprising
if the numbers given here wind up being "wrong'' by a hundred kilometers
or so after all the measurements from the eclipse season are evaluated.)

By measuring Pluto's
pull on Charon as the satellite moves around the planet, astronomers can estimate
the mass of Pluto. (Current best estimates are that Pluto's mass is about 3/1000th
that of the Earth.) From knowing the mass and the diameter, Pluto's average
density (mass per unit volume) can be calculated, giving us a due to what this
remote world is made of. On a scale where water has a density of 1.0 and the
Earth about 5.5, Pluto comes in at about 2. This means it cannot be made just
of ice, but must also contain denser (rocky) material. Knowing Pluto's approximate
mass and diameter also allows us to estimate the strength of Pluto's gravity
at its surface — it turns out to be only a percent or two of Earth's! A 100-pound
Earth student would weigh only a pound or two on Pluto.

The faint reflected
sunlight we pick up from the two bodies shows that Pluto has a surface with
a considerable amount of frozen methane (sometimes called "swamp gas''
on Earth), while Charon's surface seems to be mostly water ice. The very existence
of methane ice is an indication of how cold things must be out there — methane
does not become a solid until the temperature drops to nearly 200 degrees below
zero Celsius.

With its small
size, relatively large moon, and weird orbit, Pluto certainly seems like an
oddball world compared to the other planets in the outer solar system. One explanation
that is sometimes offered for its unusual properties is that it was once a moon
of Neptune that escaped long ago and took up an independent orbit after a while.
However, even though Pluto's orbit sometimes carries it closer to the Sun than
Neptune is, today their orbits never actually cross. This is because Pluto's
orbit is in fact outside Neptune's at the places where their planes cross. (On
the other hand, planets' orbits do change somewhat over very long periods of
time, so it is possible that Pluto's and Neptune's orbital paths did actually
cross long ago.) Furthermore, scientists are not sure what might have caused
such a moon to escape and how an escaped moon could have a satellite of its
own; so Pluto's origin must remain one of the solar system's unsolved mysteries
for now.